1. Field of the Invention
This invention relates to coating processes involving direct application of electrical, magnetic or wave energy and, more particularly, to irradiation of selected areas to produce a pattern.
2. Description of the Prior Art
Radiation-sensitive resist films are applied to substrates for the purpose of exposure to a pattern of radiation which can produce a desired pattern upon development of the resist. Resist films have been baked prior to exposure in order to remove the solvent used to apply the resist films to the substrate. It has been found that such prebaking yields cross-linking of polymers which results in poor quality images of the exposed patterns when the resists are developed.
U.S. Pat. No. 3,535,137, issued October 20, 1970 to Haller et al from U.S. Pat. application Ser. No. 609,145, filed Jan. 13, 1967 and entitled "Method of Fabricating Etch Resistant Masks" teaches a process for preparing a poly(methyl-methacrylate) resist among others using a process involving 1) dissolving a resist polymer in a suitable solvent, 2) forming a thin polymer film by spinning a drop of dissolved polymer onto a substrate surface and, 3) dry baking the polymer film for a time to a temperature sufficient to improve adhesion and handling characteristics of the film, 4) exposing selected portions of the polymer to electron beam radiation to reduce the average molecular weight of the selected portions, 5) fractionating, in situ, the selected portions by applying a developer which selectively removes the exposed polymer, 6) baking the remaining polymer for a sufficient length of time at a sufficient temperature to eliminate undercutting resulting from lateral spreading of radiation during exposure, 7) etching the underlying substrate with a desired etchant through the openings formed where the exposed polymer has been removed.
U.S. Pat. No. 3,984,582 issued Oct. 5, 1976 to Feder et al from U.S. patent application Ser. No. 591,980, filed June 30, 1975, entitled "Method for Preparing Positive Resist Image" teaches use of copolymers of a material containing polymerized methyl methacrylate with methacrylic acid (P[MMA/MAA]) as a high sensitivity resist. In this case, the process steps involve 1) coating a solution of a polymeric resist upon a substrate by spin casting or dipping and then drying to remove volatile material, 2) preferably, prebaking the polymeric film in air or in a vacuum at a temperature above the glass transition temperature of the polymeric material but below the thermal decomposition temperature of the material to remove any remaining solvent, at a temperature preferably in the 160.degree. C to 220.degree. C range, 3) exposing the coated substrate to an electron beam image with charge densities as low as 2 .times. 10.sup.-6 coul/cm.sup.2, 4) treating the polymeric film to a swelling agent to swell only the exposed portions from about 3 to 60 minutes, 5) applying a nonsolvent liquid to the polymeric film to remove the exposed portions, ultrasonically or by spraying.
One of the most important of the above steps in the preparation of the copolymers in U.S. Pat. Nos. 3,535,137 and 3,984,582 for exposure to an electron beam or other ionizing radiation is the prebaking of the resist solution only after applying it to the workpiece. The baking at temperatures of 160.degree. C to 220.degree. C changes the solubility of the copolymer in U.S. Pat. No. 3,984,582 and improves adhesion to the substrate unlike the homopolymer of PMMA in U.S. Pat. No. 3,535,137 where baking is used only for the purpose of evaporating the solvent.
The change in solubility of the copolymer resist is large and is highly dependent upon the temperature applied and the duration of its application. Accordingly, control of temperature and its duration when using this process must be very precise in order to maintain repeatability of the resist sensitivity and development times for all samples.
Ways in which changes in the P(MMA/MAA) copolymers occur during baking at temperatures below 300.degree. C are given in a publication of Jamieson et al entitled "The Thermal Degradation of Copolymers of Methyl Methacrylate with Methacrylic Acid" in the Eurpoean Polymer Journal, Vol. 10, pp. 217-225, Pergamon Press, 1975. Most of the changes involve transformation from P(MMA/MAA) to methacrylic anhydride, either by acid-to-acid reactions with the formation of water, P(MMA/MAA) ##STR1## or by ester-to-acid reactions with loss of methanol, ##STR2## These reactions result in a three-component structure, i.e., terpolymer, consisting of methyl methacrylate/methacrylic acid/methacrylic anhydride chains. Thus, baking of the copolymers produces a chemical change which produces the terpolymers. The degree to which these chemical changes occur will depend upon time and temperature.
In addition to the above reactions, main chain cross-linking can occur during baking (as shown by the dotted line), ##STR3## The latter reactions increase the molecular weight of the resist. If long chains are formed, insoluble particles are produced which remain as a residue after resist development.
In summary, baking of the copolymer resists in film form on a workpiece at temperatures from 160.degree. C to 220.degree. C (below 300.degree. C) yields chemical changes which determine the solubility of the exposed and unexposed resist in the developer solution. To achieve repeatable results from sample to sample, the baking temperature and time must be controlled very carefully. Furthermore, baking of a polymeric film on a workpiece causes cross-linking, etc., which results in producing insoluble particles which will leave an insoluble residue on the substrate even when the exposed areas are developed with a strong solvent, such as ethyl Cellosolve acetate (ECA). (Cellosolve is a trademark of Union Carbide Chemical Corporation for the mono and dialkyl ethers of ethylene glycol and their derivatives.)